Various studies have revealed that macromolecules present in the blood of patients are pathogens of some kinds of diseases such as chronic articular rheumatism and systemic lupus erythematosus. Attempts are therefore made to treat such a disease and alleviate the symptoms by removing these macromolecules from the blood of the patient. Plasma filtration processes are used as means for this purpose.
The plasma filtration processes include a treatment for separating the blood collected from the patient into blood cells and plasma by a primary filter first, and a treatment for removing the harmful macromolecules from the plasma by a secondary filter to purify the plasma. The purified plasma from the secondary filter is mixed with the blood cells from the primary filter again, and the mixture is returned to the blood supply source (patient).
The plasma filtration processes are divided generally into two types according to the mechanism of filtering off the macromolecules by the secondary filter: one is a process wherein the substance to be removed is selected according to the pore size of the secondary filter (double filtration plasmapheresis), and the other is a process comprising cooling plasma to about 4.degree. C. once to form a gel (so-called cryogel) containing the macromolecules which are the harmful components of plasma and separating off the gel with the secondary filter having relatively large pores (cryofiltration). The latter has the advantage that the harmful macromolecules can be reliably removed and therefore has attracted attention in recent years (see, for example, Examined Japanese Patent Publication HEI 1-34626).
The cryofiltration is almost similar to the double filtration plasmapheresis in mechanism with the exception of using means for cooling plasma and a secondary filter of different pore size. This process will be described with reference to FIG. 4. Blood is sent from a supply source to a primary filter 3 through a blood supply line 2 by the operation of a blood pump 1 and separated into blood cells and plasma. The separated blood cells are sent into a blood cell return line 4 so as to be mixed again with the purified plasma to be described below.
On the other hand, the plasma is sent by the operation of a plasma pump 5 through a plasma supply line 6 to a cooling coil portion 7 on the line 6 and then into a secondary filter 8, which filters the plasma. The plasma is cooled by the coil portion 7 usually to 4.degree. C. to 25.degree. C., preferably to about 4.degree. C., whereby a cryogel of macromolecules is formed. The gel is filtered off from the plasma by the secondary filter. The filtrate from the secondary filter, i.e., purified plasma, is mixed again with the blood cells returned from the primary filter 3 while being returned through a purified plasma return line 9. The mixture is thereafter warmed to the original temperature by a heating bag 10 and returned to the supply source. In the drawing, indicated at 11 are dripping chambers, at 12 pressure sensors and at A a cooling zone.
With continued plasma filtration, however, the secondary filter 8 tends to become gradually plugged and therefore needs to be restored in function by eliminating the plugging.
The plugging is eliminated usually by methods employed in the double filtration plasmapheresis. These methods include:
(a) Replacement of the secondary filter. PA1 (b) Washing of the interior of the secondary filter with physiological saline.
For example, Examined Japanese Patent Publication SHO 63-28626 discloses a method wherein two secondary filters are used as connected together in parallel, and one of the filters, when plugged, is changed over to the other. Further Unexamined Japanese Patent Publication SHO 59-129067 discloses a method wherein in the event of plugging, the internal pressure of inner chamber of the secondary filter is lowered to atmospheric pressure, and the filter is backwashed with a wash liquor supplied from the outer chamber side of the secondary filter.
With these methods (a) and (b), however, the whole amount, for example, about 200 to about 300 ml, of plasma remaining in the secondary filter is discarded by a single cycle of washing procedure. The secondary filter must be washed or replaced at least twice per cycle of plasma filtration operation, so that as much as 400 to 600 ml of plasma in total is to be discarded from the system. Consequently, the patient must be replenished with an albumin preparation and like expensive medicinal solutions, is economically burdened with an increased cost, suffers from deficiencies of substances which can not be replenished and is susceptible to side effects. Moreover, since the secondary filter is expensive, the method wherein more than one secondary filter is used results in a correspondingly increased treatment cost.
Especially with the cryofiltration wherein the secondary filter used has a relatively large pore size, the filtration efficiency is lowest at the start of filtration and gradually increases with accumulation of cryogel. Thus, this process has the problem that the filtration efficiency of the secondary filter markedly decreases every time the filter is replaced or washed.